Operating system I/O schedulers attempt to order and merge I/O requests to increase the overall I/O throughput across a set of devices from a set of concurrent processes. One aspect of some of these schedulers is to introduce a delay to a device in anticipation of additional I/Os that can be ordered and/or merged with previous I/Os. This is especially helpful, in theory, four subsequent workloads were mergable candidates are abundant.
The drawback to this approach is that the delay is typically time-based and does not take into consideration the current load that is already enqueued in the device. Also, in high-process count environments, single device queue algorithms lose potential candidates as the I/Os are spread to far apart.